<!DOCTYPE html>
<html lang="en" xmlns:v-on="http://www.w3.org/1999/xhtml">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
</head>
<body>

<div id="app">
  <h2>{{count}}</h2>
  <button v-on:click="incr()">+</button>
  <button v-on:click="decr()">-</button>
</div>

<script src="../js/vue.js"></script>
<script>
  const app = new Vue({
    el: '#app',
    data: {
      count: 0
    },
    methods: {
      incr: function () {
        this.count++;
      },
      decr: function () {
        this.count--;
      }
    }
  })
</script>

</body>
</html>